1. Field of Invention
This invention relates to a system that measures weight in a seat of a motor vehicle, weight sensors or strain transducers used in the system and the calibration of the weight sensors. In particular, the invention relates to a method and system of calibrating weight sensors used in motor vehicles and to a transducer that can be used as a weight sensor in the system.
2. Description of Prior Art
Weight sensors are used in a motor vehicle seat to measure strain or stress as a result of weight in the seat. The amount of measured stress is processed with other information, such as in seat occupant positioning, to control primary and supplemental restraint systems, such as managed load limiters, pre-tensioners and/or side and frontal air bags. Accurate weight in seat information combined with supporting primary and supplemental restraints improves overall system decisions and performance of the system in protecting occupants of the vehicle should a crash occur.
Current system installations use on-line assembly and field personnel to calibrate the system. These systems use data sampling, performance history data and a one-time zero set or reference to achieve initial and ongoing calibrations. The one-time calibration is usually performed at the time of initial installation in the vehicle. This type of calibration is unlikely to capture a shift in zero set as a result of over stressing of the weight sensor during vehicle use, aging of components, and other drift causing factors that occur throughout the lifetime of the vehicle. Thus, such one-time calibration systems will over time result in a shift in weight measurement readings that increase the system error rate as the vehicle ages.
Weight sensors used for sensing weight in the seat of a motor vehicle have included pressure or bladder units, flexible membrane units, proximity sensor units or structural beam units, that generate a change in an electrical characteristic such as resistance or capacitance. These units typically include an elastomerically deformable element upon which is mounted a plurality of strain gauge elements. Each of the strain gauge elements has an electrical characteristic, such as resistance, capacitance or inductance that varies as the elastomeric element deforms under stress.
Conventional elastomeric beam transducers have generally been formed with a bending process. Stress is distributed by such transducers in a central region centered on a fore/aft axis, but offset from a lateral axis thereof. This has required the use of two strain gauge elements to capture positive and negative stress loads. Two strain gauges have complicated the system tasks of measuring weight, compensation, and calibration. This affects the complexity and cost of system hardware and software.
Thus, there is a need for a weight sensing system for a motor vehicle that has the capability of providing a calibration procedure and system that improves accuracy of the system over the lifetime of the vehicle. There is also a need for a weight sensor that distributes stress in a central region that is centered about a fore/aft axis, but without an offset from a lateral axis thereof.
A strain transducer according to the invention includes a body having two lands with an elastomeric beam in juxtaposition with the two lands. The mass of the elastomeric beam is less than the mass of either of the lands. The elastomeric beam has a region of minimum thickness and one or more regions of maximum thickness. An electrically resistive body is located on the elastomeric beam overlying the region of minimum thickness. First and second electrical contacts electrically contact spaced part locations of the resistive body, whereby deformation of the elastomeric beam results in a change in electrical resistance of the electrically resistive body between said first and second electrical contacts.
In some embodiments one or both of opposed surfaces of the elastomeric beam are arcuate. In some embodiments, the electrically resistive body is a thick film resistor that is adhered to an arcuate surface of the elastomeric beam.
In another embodiment, the strain transducer includes a resistive body disposed on an elastomeric beam with four equally spaced electrical contacts disposed thereon to define four electrical resistances of the body that are connected in a wheatsone bridge.
A method according to the invention automatically calibrates a characteristic of a weight sensor installed to sense weight of a seat in a motor vehicle that has a door and a seat belt with a buckle associated with the seat. The method determines if the seat is empty by means of the weight sensor, a spatial detector or a pressure sensor or the like. Next, the method determines if a condition is present, where the condition is a member of the group consisting of the door being open and the seat belt being unbuckled. If the seat is empty and the condition is present, a set of values of the characteristic that define a calibrated zero set value are established. The calibrated zero set value is then presented for use as a zero set for in seat weight measurements. The determining step, the establishing step and the presenting step are repeated for as long as the vehicle is in use.
More particularly, the method retains a first set of the established values as a reference set of values and a second set of values as a temporary set of values, if the second set of values exceeds a predetermined deviation from the reference set of values. The temporary set of values defines a modified zero set value reflecting drift of the weight sensors for the in seat weight measurements.
In a particular embodiment, a plurality of weight sensors are installed to sense the weight of the seat and the sets of values are derived from measurements of the electrical resistance of the plurality of weight sensors. According to one feature of the invention, the number of times a temporary set of values is established is limite during the time an open door condition is found present.
The weight measurement system of the invention includes a computer controller that performs the method of the invention.